To remove liquid and the substances that must be eliminated in urine, various methods of purifying and/or treating blood with machines have been used to treat chronic renal failure. Diffusive mass transport is predominant in hemodialysis (HD), and convective mass transport through the membrane occurs in hemofiltration (HF). Hemodiafiltration (HDF) is a combination of these two methods. In peritoneal dialysis (PD), no extracorporeal circulation is needed and the peritoneum is used as a contact membrane.
Because of the large exchange quantities, it is necessary to accurately balance the fluid withdrawn, on the one hand, with the fluid supplied, on the other hand, as well as the volume to be subjected to ultrafiltration over the total treatment time with the known methods an also with continuous arteriovenous HF, continuous venovenous HF and plasma filtration (PF). Gravimetric and volumetric balancing systems are known in the prior art.
In addition, there are also known methods, which measure the fluid flows of the fluids flowing into the dialyzer and the fluids flowing out of the dialyzer continuously and balance them with respect to one another. Flow measurement sensors or flowmeters of various designs are used for this purpose.
Magnetic flowmeters, which are also known as electromagnetic flowmeters or inductive flowmeters are based on the measurement of the velocity of flow of a conductive fluid through a known or controlled magnetic field by measuring the induced electric voltage. With a known flow cross section, the flow rate or the volume flow can be deduced from the velocity of flow, and then must be covered by the concept of fluid flow in the following discussion. An electric voltage occurs in a magnetic field through which a flow passes through a charge separation of the ions which are present in a conductive fluid and can be measured as the induced voltage. The voltage measurement is typically performed by deriving the induced voltage from a pair of electrodes which in electric contact with the conductive fluid or which are linked capacitively to the fluid. This voltage is proportional to the velocity of flow and depends on the magnetic field strength. The charge separation takes place in the direction perpendicular to the direction of flow and to the direction of the magnetic field. The magnetic field of a magnetic flow mater is therefore preferably disposed perpendicular to the direction of flow in the corresponding fluid channel, and the electrode pair for diverting the induced electric voltage is preferably disposed perpendicular to the magnetic field and also perpendicular to the direction of flow in the fluid channel.
A typical electromagnetic flowmeter is constructed from a nonmagnetic and nonmagnetizable pipe, which is lined with an electrically insulating material on the inside.
The magnetic field is typically generated by one or more coils disposed outside of the tube through which the fluid flows. The electric voltage induced by the fluid flow is typically determined by a voltmeter. The result of the voltage measurement is sent to an evaluation unit for determining the fluid flow, i.e., the flow rate or the volume flow based on the measured voltage.
If an electromagnetic flowmeter is designed as a differential flowmeter for measuring a flow difference between a first and a second fluid-carrying channel, then advantageously a joint magnetic field will penetrate through the first and second fluid-carrying channels.
If the first and second fluid-carrying channels correspond to one another with regard to their geometric dimensions, then the voltage difference between a first electrode pair disposed on the first fluid-carrying channel and a second electrode pair disposed on the second fluid-carrying channel indicates directly the difference between the flow in the first channel and the flow in the second channel. If the first and second electrode pairs are connected in series, this voltage difference can be picked up directly.
An electromagnetic differential flowmeter is advantageously constructed from the one cassette module in which the fluid-carrying channels are formed, each having an electrode pair, an electromagnet or permanent magnet for generating a magnetic field between the electrode pairs and an evaluation unit for evaluating the voltages or the differential voltage between the electrode pairs.
The cassette module in which the fluid-carrying channels are formed is advantageously designed as a disposable part, for example as part of a dialysis fluid circulation.
The cassette module may or may not have additional elements of the dialysis fluid circulation as part of a dialysis fluid circulation.
The inventors of the present invention have recognized that a temperature difference between the first fluid-carrying channel and the second fluid-carrying channel can lead to a reduced precision of the differential flowmeter.
Therefore the object of the present invention is to provide improved balancing of the fluid flows.